Remember basic arithmetic from school? These work just
Opposite of $a.
$a + $b
Sum of $a and $b.
$a - $b
Difference of $a and $b.
$a * $b
Product of $a and $b.
$a / $b
Quotient of $a and $b.
$a % $b
Remainder of $a divided by $b.
The division operator ("/") returns a float value unless the two operands
are integers (or strings that get converted to integers) and the numbers
are evenly divisible, in which case an integer value will be returned.
Operands of modulus are converted to integers (by stripping the decimal
part) before processing.
Remainder $a % $b is negative for negative